1. Field of the Invention
The present invention relates to an anode material comprising nearly spherical graphite particles and crystalline carbon covering the graphite particles. More particularly, the present invention relates to an anode material which has a large capacity, a high potential and an excellent charge-discharge cycle property and which causes no decomposition of electrolytic solution; a process for production of such an anode material; and a lithium secondary battery using such an anode material.
2. Description of the Related Art
As electronic appliances have become smaller and lighter, the batteries used therein are required to have a high energy density. It is also required to develop a high-performance secondary battery allowing repeated charge and discharge, from the standpoint of resource saving.
In order to meet such requirements, lithium secondary battery was proposed and development thereof is being continued.
Lithium secondary battery is classified, based on the kind of electrolyte used therein, into lithium ion secondary battery, lithium polymer secondary battery, solid lithium secondary battery, etc. Currently, lithium ion secondary battery is used most widely in practical applications.
As the anode of lithium ion secondary battery, there are a graphite-based anode and a carbon-based anode. The graphite-based anode requires a short time for charging and is practical and shows a high coulomb efficiency; therefore, the graphite-based anode is a main stream currently.
The graphite-based anode has had problems in that (a) the graphite reacts with a solvent constituting an electrolytic solution, which reduces the coulomb efficiency of battery and (b) the graphite decomposes the solvent to give rise to generation of a gas. The particularly serious problem possessed by the graphite-based anode is that the graphite decomposes propylene carbonate (hereinafter abbreviated to PC) having an excellent solvent property at low temperatures, making it difficult to use PC for electrolytic solution.
To solve this problem, it was proposed to cover the surfaces of graphite particles with low-crystallinity carbon or amorphous carbon, each causing no PC decomposition.
Meanwhile, the present inventors previously found out that the above problem could be solved by uniformly covering the whole surfaces of graphite particles with crystalline carbon, and filed a patent (JP-A-2000-106182). The technique disclosed in the literature relates to an anode material which is a graphite-carbon composite material obtained by uniformly and completely covering the surfaces of graphite particles with crystalline carbon by chemical vapor deposition. When this anode material is used for production of an anode for lithium ion secondary battery, the anode obtained can reliably prevent the decomposition of PC or the like.
The reason for prevention of the decomposition of PC or the like is presumed to be that since the surfaces of graphite particles are covered with the [002] plane of crystalline carbon, the infiltration of solvent into anode material and resultant contact of solvent with graphite particles are prevented.
A battery produced using this anode material has a large discharge capacity and allows quick charge. Therefore, this anode material, as compared with conventional anode materials covered with low-crystallinity carbon, has superior electrode properties.
The anode of lithium ion secondary battery is produced by adhering an anode material to the surface of a conductive collector using a small amount of a binder, to form a thin anode material layer on the collector. The anode is subjected, during the production, to a treatment for increasing the density of the anode material layer, in order to obtain a large battery capacity. Specifically, the anode material layer is compressed using a means such as pressing, rolling or the like, to increase the density of the anode material layer (this density is hereinafter referred to as electrode density).
The graphite used in the above anode material (the graphite-carbon composite material) may be natural graphite or artificial graphite. Being inexpensive, natural graphite is economically superior to artificial graphite.
The graphite-carbon composite material as anode material, produced using natural graphite, however, has a high mechanical strength and therefore hardly causes deformation; hence, the composite material has a problem in that no sufficiently high electrode density is achieved by a means such as pressing, rolling or the like.
Further, the graphite particles obtained by grinding natural graphite are scaly in shape.
Natural graphite is fundamentally formed in a structure in which a large number of networks of carbon atoms, i.e. a large number of AB planes are laminated in a large thickness in the form of a lump. The bonding force between AB planes (the bonding force in the C axis direction) is far smaller than the bonding force within each AB plane. Therefore, in grinding of natural graphite, peeling between AB planes takes place preferentially unless a special countermeasure is taken in the grinding, and the resulting graphite particles are scaly.
Scaly graphite particles have a large specific surface area; therefore, they need a larger amount of carbon when a carbon layer is formed thereon to produce a graphite-carbon anode material. Use of a larger amount of carbon for carbon layer formation results in an anode material of higher strength; as a result, the anode material is resistant to deformation. This is because while graphite particles are relatively soft, the carbon layer covering them has a high mechanical strength. The fact that no sufficiently high electrode density is obtained, holds also when carbon of low crystallinity is used in the carbon layer.
Artificial graphite can be produced as nearly spherical particles, depending upon the production process. It can also be produced as particles low in anisotropy. Artificial graphite can be produced, for example, as spherical graphite particles having a structure in which disc-shaped graphite plates different in radius are laminated in the C axis direction, or as columnar graphite particles having a structure in which disc-shaped graphite plates equal in radius are laminated in the C axis direction.
Such artificial graphite, however, is generally expensive and low in crystallinity. Use of artificial graphite low in crystallinity as an anode material is not preferred because such graphite is small in charge and discharge capacities per unit mass of graphite.
Meanwhile, artificial graphite high in crystallinity has properties close to those of natural graphite. Therefore, artificial graphite high in crystallinity, when ground, becomes scaly (plate-like) particles as in the case of natural graphite.